The present invention relates to the propulsion of undersea vehicles and especially to a propulsion system for such vehicles which, in its low speed mode, combines backflushing with propulsion.
When an undersea vehicle such as a torpedo travels through the ocean, surface friction creates a boundary layer of retarded fluid next to its surface. The boundary layer at the extreme forward region of the vehicle nose is smooth and "laminar" in character, but quickly changes to "turbulent" as the flow progresses rearward from the nose. Thus, nearly all of the vehicle boundary layer is normally turbulent. The friction or drag of a turbulent boundary layer is seven to ten times that of a laminar boundary layer, so the possibility of achieving drastic reductions in vehicle drag and power by maintaining a laminar boundary layer is an attractive challenge for the engineer and scientist. The principle of maintaining laminar flow by surface suction applied through many fine circumferential slots on a very smooth body of revolution has been demonstrated in a wind tunnel, and the configuration has been described in U.S. Pat. No. 3,604,661. But subsequent experiments on suction laminar flow control for a buoyant body propelled in the ocean have demonstrated inadequate laminarization. It is hypothesized that the suction laminar flow control ocean vehicle captured ocean particles at the slot entrance in the low speed regime, and that the captured ocean particles remained in the slots, protruding into the boundary layer, and thus caused transition to turbulent boundary layer flow. It is known from subsequent experiments that, as the speed progresses, a speed is reached above which no ocean particles can be captured by the flush slots because forces on the particles and the momentum of the particles sweep them on downstream past the slots. In the low speed regime, however, backflush (blowing rather than suction through the slots) is required to prevent the capture of ocean particles. Thus, a system is required that will provide backflush in the vehicle low speed range of zero to approximately one fifth the vehicle "maximum" speed, and at the same time providing thrust to accelerate the vehicle. Such a system is the subject of the present invention.